An energy storage device, especially a lithium secondary battery, has been widely used recently for a power source of a small-sized electronic device, such as a mobile telephone, a notebook personal computer, etc., and a power source for an electric vehicle or electric power storage.
There is a high possibility that a battery mounted on such an electronic device or a vehicle is used at a high temperature in midsummer or in the environment warmed by the heat generation of the electronic device. With respect to a thin electronic device, such as a tablet device, an ultrabook, etc., a laminate-type battery or a prismatic battery using a laminate film, such as an aluminum laminate film, etc., for an outer packaging member thereof is frequently used. In such a battery, the outer packaging member is thin, and therefore, there is involved such a problem that the battery is liable to be deformed due to a bit of expansion of the outer packaging member, or the like, so that the deformation very likely influences the electronic device.
A lithium secondary battery is mainly constituted of a positive electrode and a negative electrode, each containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution containing a lithium salt and a nonaqueous solvent; and a carbonate, such as ethylene carbonate (EC), propylene carbonate (PC), etc., is used as the nonaqueous solvent.
In addition, a metal lithium, a metal compound capable of absorbing and releasing lithium (e.g., a metal elemental substance, a metal oxide, an alloy with lithium, etc.), and a carbon material are known as the negative electrode of the lithium secondary battery. In particular, a nonaqueous electrolytic solution secondary battery using, as the carbon material, a carbon material capable of absorbing and releasing lithium, for example, coke or graphite (e.g., artificial graphite or natural graphite), etc., is widely put into practical use.
Since the aforementioned negative electrode material stores and releases lithium and an electron at an extremely electronegative potential equal to the metal lithium, it has a possibility that a lot of solvents are subjected to reductive decomposition especially at a high temperature, and a part of the solvent in the electrolytic solution is reductively decomposed on the negative electrode regardless of the kind of the negative electrode material, so that there were involved such problems that the movement of a lithium ion is disturbed due to deposition of decomposed products, generation of a gas, or expansion of the electrode, thereby worsening battery characteristics, such as cycle property, etc., especially in the case of using the battery at a high temperature; and that the battery is deformed due to expansion of the electrode. Furthermore, it is known that a lithium secondary battery using a metal lithium or an alloy thereof, a metal elemental substance, such as tin, silicon, etc., or a metal oxide thereof as the negative electrode material may have a high initial battery capacity, but the battery capacity and the battery performance thereof, such as the cycle property, may be largely worsened especially at a high temperature because the micronized powdering of the material may be promoted during cycles, which brings about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode formed of a carbon material, and the battery may be deformed due to expansion of the electrode.
Meanwhile, since a material capable of absorbing and releasing lithium, which is used as a positive electrode material, such as LiCoO2, LiMn2O4, LiNiO2, LiFePO4, etc., stores and releases lithium and an electron at an electropositive voltage of 3.5 V or more on the lithium basis, it has a possibility that a lot of solvents are subjected to oxidative decomposition especially in the case of using the battery at a high temperature, and a part of the solvent in the electrolytic solution is oxidatively decomposed on the positive electrode regardless of the kind of the positive electrode material, so that there was involved such a problem that the movement of a lithium ion is disturbed due to deposition of a decomposed product or generation of a gas, thereby worsening battery characteristics, such as cycle property, etc.
Irrespective of the foregoing situation, the multifunctionality of electronic devices on which lithium secondary batteries are mounted is more and more advanced, and power consumption tends to increase. The capacity of the lithium secondary battery is thus being much increased. Because of an increase of a density of the battery, a reduction of a useless space capacity within the battery, and so on, a volume occupied by the nonaqueous electrolytic solution in the battery is becoming small. In consequence, it is the present situation that the battery performance at a high temperature is apt to be lowered by decomposition of a bit nonaqueous electrolytic solution.
PTL 1 discloses a nonaqueous electrolytic solution for a secondary battery having a lithium salt dissolved in a nonaqueous solvent, which contains lithium bis(oxalate)borate as the lithium salt, further contains at least one compound selected from a compound having an S—F bond, a monofluorophosphate, a difluorophosphate, and the like in an amount of 10 ppm or more in the whole of the nonaqueous electrolytic solution, and still further contains LiPF6. PTL 1 describes that the output characteristics, high-temperature storage property, and cycle property are excellent.
PTL 2 discloses an electrolytic solution for a nonaqueous electrolytic solution battery composed of a nonaqueous organic solvent and a solute, which contains, as additives, at least one compound selected from a first compound group having an oxalate structure, which is composed of difluoro(oxalate)borate, difluoro(bisoxalate)phosphate, and the like, and at least one compound selected from a second compound group composed of a monofluorophosphate and a difluorophosphate. PTL 2 describes that the cycle property and durability can be improved.
PTL 3 discloses a nonaqueous electrolytic solution containing a nonaqueous solvent, LiPF6, and a specified fluorosulfonate, in which a ratio of the molar content of FSO3 to the molar content of PF6 is 0.001 to 1.2, and which further contains 0.0005 to 0.5 mol/L of a lithium fluorophosphate. PTL 3 describes that the initial discharge capacity, impedance characteristics, and output characteristics are excellent.
PTL 4 discloses an electrolytic solution for a nonaqueous electrolytic battery containing a nonaqueous organic solvent and a solute, which contains, as additives, difluoro(bisoxalate)phosphate and tetrafluoro(oxalate)phosphate together. PTL 4 describes that the cycle property, high-temperature storage property, and low-temperature properties can be improved.